Chocolate bars are the third type of currency in Candy Box, next to candies and lollipops. You can get one from the Frog who lives in the Swampy Swamp. More can be found in the computer tab via the bugs, as well as by cheating via the Javascript/Developer Console.
After the player attains more than one chocolate bar, the text will change: "You have 2 chocolate bars \o/ (a bug ? Oo)"